The present invention relates to a method of separating air by a low temperature rectification process employing a distillation column to produce a nitrogen product. More particularly, the present invention relates to such a method and apparatus in which a portion of oxygen enriched liquid produced as a column bottoms in the distillation column vaporized and then expanded to supply refrigeration and another portion of the oxygen enriched liquid, after expansion, is used to condense nitrogen vapor in a head condenser attached to the distillation column. Even more particularly, the present invention relates to such a method and apparatus in which the portion of the oxygen enriched liquid is vaporized by a part of the incoming air and under certain conditions an additional condensing stream of lesser oxygen content than air withdrawn from the distillation column. The part of incoming air and the additional condensing stream are thereby liquefied and introduced into the column as additional reflux streams to maintain the rate and/or concentration of nitrogen production to prior art levels.
Nitrogen is produced by low temperature rectification of the air in an air separation plant. Often such plants employ a single distillation column and are known in the art as nitrogen generators. After air has been filtered, compressed and purified, the air is cooled to a temperature suitable for its rectification. This temperature is normally at or near the dew point of the air. Thereafter, the air is introduced into a distillation column having liquid-vapor contacting elements which can be formed by trays and/or packings, either structured or random. In the distillation column an ascending vapor phase of the air is contacted by a descending liquid phase. The result of such contact is that the liquid phase becomes evermore concentrated in oxygen to produce a oxygen enriched liquid column bottoms and the ascending vapor phase becomes evermore concentrated in nitrogen to produce a nitrogen rich vapor tower overhead.
In order to reflux the column, a head condenser is provided in which the nitrogen vapor tower overhead is partially condensed. The condensate is returned to the distillation column as reflux. Typically, an oxygen enriched liquid stream composed of the column bottoms is removed, expanded to a low temperature, and then introduced as the coolant for the head condenser. The product is removed from the top region of the column mostly as a vapor.
In any type of air separation plant, there is continual heat leakage into the plant and enthalpy differences between the air feed and product streams at the warm end of the plant. Such heat leakage requires refrigeration to be supplied to the air separation plant. If the nitrogen product is to be maintained at column pressure, refrigeration is generally supplied from outside the column envelope. Work expansion obtained from the vaporized oxygen enriched liquid, all of which is vaporized in the head condenser, or by expanding air from a higher pressure down to column pressure are usual methods of supplying refrigeration. There are also "liquid assist plants" in which liquid nitrogen is added to the column from an external source in order to supply the requisite refrigeration.
As will be discussed, the present invention relates to an air separation technique in which refrigeration is generated in a manner that reduces the energy expenditure in producing a nitrogen product. This is accomplished by more efficiently using energy for air separation and making energy, formerly in excess, available for refrigeration.